Method of chromium plating



higher strength may bealso-plated.

United States Patent METHOD OF .CI-IROMIUM PLATING Jesse E. Stareck,Royal Oak, Edgar J." Seyb,'*Jr.; Oak Park, and Frank Passal, Detroit,Mich.,- assignorsyby 'mesne assignments, to Metal & Thermit Corporation,New York, N. Y., a corporation of New Jersey 'No Drawing. Application"July 26, 1955, :Serial No;524',578

11 Claims. "(CL 204-51) *This "invention relates to an improved methodfor chromium plating 'hard', high strength steels. Conventional chromiumplating of such-steels is accompanied by such ahigh loss of fatiguestrength that failure ofthe steel usually occurs under repeated loading;moveover, the loss in'fatigue strengthincreases with increasing strengthof the steel. *This unfavorable effect of conventional chromiumplatinghas naturally limited the usefulness of these plated steels.

A-principal object or the inventionis to provide a method forchromium"platingh'ard, high strength steel without substantial loss'offatigue strength. "The method is particularlydirected-to the'platingofsteel articles for use at ordinary temperatures under conditions ofrepeated stress and strain that approach the fatigue limit of the steel.It has been found that ahigh fatigue strength may be obtained in theplated article if the chromium deposited on-it contains a large numberof fine cracks such that it exhibits a low residual stress, Theresultant chromium plated steel enjoys the protection against corrosionand -wear, and the hardnessand low resistance to friction,

- a. s. i., is passed-to the article as cathode inanaqueous bath havinga chromic acid, CrOz, content of 100 to 400 .g./l., preferably l50to-300 g./l.; and containing dissolved sulfate, SO4=,,as catalyst. -Theratio of 0193 to .dissolved S04 is 40:1 to 100: 1, and the bathtemperature is 90 to 110 F. A wider temperature range, namely, 90-to 130F., may be employed for baths having a CrO3/SO4 ratio of 40:1 to85 :1.Chromium is. plated on the article to forma coating at least 1 milthick.

The chromic acidmay be supplied to the bath as such and the sulfateinthe form of anysuitable compound, such as sulfuric acid.

At CrOs concentrations f300 to 400 g./l. the bath may be made to beself-regulating with respect to the sulfate concentration. Such a bathcomprises CIO3 and, as the source of the sulfate, calcium sulfate in anamount sufficient to saturate the bath and to provide an undissolvedexcess of calcium sulfate. As this saturation concentration of dissolvedsulfate is higher than required; a soluble calcium compound is added inan amountsufficient to suppress the concentration of dissolved sulfatefrom its unsuppressed saturation concentration to a "lower valuecorresponding to the required CrO3/SO4 ratio. Thus, as dissolved sulfateis removed from the'bath, as by drag out losses, sufiicient excesscalcium sulfate goes into solution to replace the lost sulfate. Lowertempera tures and ratios are preferred for this type of bath; thus, inthe case-of a CrO3 concentration of 300 g./l., a ratio of 40:1 to 60:1is quite satisfactory. Suitable soluble ,g./l. for a ratio'of to 1.g./l. of CrOa a 40 to 1 ratio-and a 60 to 1 ratio canbe '2 calciumsuppressor compounds for providing dissolved calcium are calcium oxide,calcium .hydroxide,'calcium carbonate, calcium dichromate, and calcium,chromate. For example, for baths ontaining 400 g./l. of CrOs there isadded sufi'icient calcium sulfate to the solution to saturate it andthen 7'g./l.'of'dissolved suppressor calcium are added to give a ratioof CrOa-to S04 of 40 to 1; 14 g./l. of dissolved suppressor calcium areadded to give a ratio of 60 to 1; 20 g./l. for a ratio of 80 to 1; and25 For baths containing 300 obtained with 12' and'20 g./l.,respectively, of dissolved suppressor calcium.

The unplated steel mayhave a Rockwell hardness, of at least 20C. Theupper limit of hardness. may be'as desired, for example up to 54C. Goodresults .are obtainablev With steels .havinga Rockwell hardness in'therange of 28C..to 47C, particularly. 33C to-47C. Alloy or high quality:steels are quite suit-able. for plating, but theinvention is notlimitedto them. Substantial thicknesses of chromium are deposited on the steel,ranging from 1 or 2 to 30 or. more mils, and preferably from 5 to. 15mils. Plating .times may very widely, for example 1 from 1.to24 ormorehours.

[Chromium 'electrodeposits exhibit the characteristioof stress, aproperty Which..changes with the thickness of the depositand Which,.bymeans of. a spiral contractometer, can be measured throughout the courseof afplating operation. With .this instrumentjthe stress in a longcontinuous section ofchromium plateis transferredto the underlying basemetal comprisingalong, helically wound strip of thin metal havingone end.free to rotate. vThe stress in the plate can be calculated from theamount. of rotation. It has been foundthat initially, the stress in adeposit increases very rapidlyduring the first'few mo- -ments ofplatinguntil it exceeds the tensile strength. of

the deposit, at which point crack-lines form inthe chr0- 'miurn torelieve'the high tensile stress. As thethickness guished. from the.positive values previously obtained. This behaviour is illustrated inTable 1 belowwhich sets forth stress data, as obtained by the spiralcontractometer,

at various plate thicknesses for a bath whose composition and operating.conditions aregiven in Example? of Table 3. It may. benotedth'at intheplating run carried out at2 a. s. i., the stress was already dropping bythe time the first readings on the contractometer were taken.

Table 1 Stress, 1,000 p.s.i. Chromium thickness, mils 1 a.s.i. i 2a.s.i.

0.025 161 175 0.05 .166 159 0.1-- 156 157 0.2 106 0.3 74 87 0.4-- 54 64I 0.5 42 47 0.75- 26 31 1.0 v 17 20 1.5 8 =10 2.0 3 5 2.5 1 2 3.0.- 0 o.5 3.5.- 0.5 4.0... ?1 1 i of -15,000 or -20,000 p. s. i.

'- 1-8) and exhibited the properties listed in Table 2. iEach depositwas 2 mils thick. The three deposits were produced from baths describedin Examples 7, 1 and 6.

Table 2 ,5 from measured stress, and conversely, the fewer thecracklines, the higher the stress. The correlation applies over a widestress range, varying from a positive tensile stress of about +80,000 p.s. i. to a negative compressive stress Between about -15,000 and 20,000p. s. i. the stress tends to level off such that further increases inthenumber of crack-lines is not accompanied by more negative stressvalues. The correlation holds for thicker deposits, say those of atleast 1 mil thickness. By counting the number of crack-lines in adeposit it is possible to obtain an indirect semi-quantitative measureof the stress in the deposit and to compare one deposit with another.

The number of crack-lines in chromium plate deposited on hardened highstrength steel is also correlated with the fatigue strength of theplated steel, the more numerous. the crack-lines, the higher being thefatigue strength. It follows that deposits exhibiting high tensilestress result in a severe lowering of the fatigue strength of the steel,whereas deposits of low stress give much less lowering of the fatiguestrength. It is desirable to deposit on the steel a plate having 1000,preferably 1200, to 10,000 crack-lines per inch. In terms of stress, theplate should range from +5000 p. s. i. tensile stress, preferably p. s.i., to -20,000 p. s. i. compressive stress. Steel plated with depositsof this character exhibit a higher fatigue strength than if plated withdeposits not so characterized. Fatigue strengths of about 70%, and more,of the unplated steel are obtainable, with strengths of at least 85%being preferred. Strengths of 60 to 70% of i the unplated steel aresatisfactory for many purposes.

The crack pattern of a deposit'is determined early in the platingoperating, say after about 0.5 mil of chromium is deposited. Aconsequence of this fact is that an article may be plated to a pointwhere a desirable crack pattern, i. e., numerous crack-lines, has beenestablished a and then transferred to another bath of the same type butoperated at a higher speed, so that a desirable plate of requiredthickness may be obtained in a shorter time. Another characteristic ofthe method is that the crack pattern of a deposit maybe favorablychanged during plating as by periodically reversing the current.

An incident of the chromium plating of steel in chromic acid platingbaths is the embrittlement of the steel by hydrogen released during theplating operation. Where embrittlement of the chromium plated steel is afactor to be considered in connection with the end use 'of the steel, itcan be prevented or substantially minimized by applying a barrier metalsuch as a copper lstrike between the steel and the chromium. Copperstrike films may be applied by plating the steel in a suitable copperplating bath for a short time to deposit about 0.01 to 0.1 or, 0.2 milof copper. The fatigue strength of the resulting copper plated, chromiumplated steel, by comparison with a like steel similarly chromium platedbut not copper plated, is not affected by the presence ofthecopper'barrier.

, Suitable copper plating baths include conventional cyanforming, Blumand Hogaboom, revised 3rd edition, 1949, pages 295 and 290,respectively; and conventional pyrophosphate copper baths,as describedin U. S. Patents 2,250,556, 2,437,865, and 2,493,092.

The invention may be illustrated by the examples which follow, showingseveral plating baths, some within and some outside of the invention,their operation, and crack properties of the plate produced by thebaths.

EXAMPLES 1 TO 8 A number of baths, the compositions of which are listedin Table 3, were made up from chromic and sulfuric acids to provide theClO3/S04 ratios noted. Standard steel fatigue test specimens were platedin the baths at the temperature and current density conditions setforth. The specimens were the same, comprising onehalf inch round barstock of SAE 4140 steel. Each specimen was 3% long and had a centralportion that was reduced to a diameter of A". The specimens had beenheat-treated and had a hardness of Rockwell C43. The fatigue strength ofthe specimen steel was 109,000 p. s. i. and the tensile strength was222,500 p. s. i. Crack-lines were determined by microscopicallyexamining the surface of a specimen at 600K: an arbitrary straight lineone inch long was selected in the microscopic field and the averagenumber of crack-lines, including filled-in lines, which crossed sucharbitrary line was counted.

Table 3 Exam. CrOa, Ratio: Temp., 0. D., Thick- Crack- No. g./l. CrOi toF. a. s. i. ness, lines per SO mils in.

250 100 130 2 2 400 250 130 2 6 to 7 1, 200 250 70 130 2 6 t0 7 1, 200250 55 130 2 6 to 7 1, 400 250 40 130 2 6 t0 7 1, 600 250 150 2. 9 2 50250 100 130 2 2 400 250 100 2 2 1, 200 250 40 130 2 6 to 7 1, 600 250 402 6 to 7 1, 600

As shown by Examples 1 to 5, as the CrOs to sulphate ratio is decreased,the number of'crack-lines per inch increases, the temperature beingconstant. If the ratio is held contant, and the temperature isdecreased, the number of crack-lines increases, as shown by Examples 6,1 and 7, and also by Examples 5 and 8. In these last two groups ofexamples, the listing of Examples 1 and 5 are repeated merely toillustrate the relationship more readily. Example 1 represents astandard or conventional sulphate bath operated at standard conditions,and as is apparent, it does not produce a plate having the desirednumber of crack-lines. Example 6, like Example 1, also illustrates aplate not within the invention, the temperature in both examples beingtoo high. Fatigue strengths of specimens plated according to Examples 1and 8, except that thicker deposits were plated, are compared in Example9.

EXAMPLE 9 Test specimens of the kind described were plated according toExamples 1 and 8, except that thicknesses of 10 and 11.6 mils,respectively, were deposited on the specimens. Fatigue strengths of theplated specimens were determined with a rotating beam fatigue machine ofthe R. R. Moore type. The specimen plated according to Example 1 had afatigue strength of 57,000 p. s. i., while that plated according toExample 8-had a strength of 67,500 p. s. i.

Unless otherwise specified, the reference in the claims to the Cl'Oscontent of the baths is intended to include the amount ofv CrOs addedper se and the amount added in the form of a chromate or dichromatesalt.

In the light of the foregoing description, the following is claimed:

1. A method for reducing the loss of fatigue strength following chromiumplating of an article of hard steel comprising electrodepositingchromium on the article by passing a current of A to 5 a. s. i. to saidarticle as cathode in an aqueous chromium plating bath at a temperatureof 90 to 110F., said bath having a CrOa content of 100 to 400 g./l. andcontaining dissolved sulfate, SO4=, as catalyst, the ratio of CrOs todissolved sulfate being 100:1 to 40:1, plating chromium on the articleto form a deposit at least 1 mil thick, said deposit being characterizedby having at least 1000 crack-lines per inch and a stress below +5,000p. s. i., and thereby producing a chromium plated article of hard steelhaving a fatigue strength of at least 60% of that of the originalunplated steel.

2. A method for reducing the loss of fatigue strength following chromiumplating of an article of steel having a hardness of at least 20CRockwell and a fatigue strength of at least 50,000 p. s. i., comprisingelectrodepositing chromium on the article by passing a current of A to 5a. s. i. to said article as cathode in an aqueous chromium plating bathat a temperature of 90 to 110 F., said bath having a CrOs content of 100to 400 g./l. and containing dissolved sulfate, SO4=, as catalyst, theratio of C103 to dissolved sulfate being 100:1 to 40:1, plating chromiumon the article to form a deposit about 1 to 30 mils thick, said depositbeing characterized by having 1000 to 10,000 crack-lines per inch and astress of +5,000 to 20,000 p. s. i., and thereby producing a chromiumplated article of hard steel having a fatigue strength of at least 60%of that of the original unplated steel.

3. A method for reducing the loss of fatigue strength following chromiumplating of an article of steel having a hardness of 33C to 47C Rockwelland a fatigue strength of 70,000 to 110,000 p. s. i., comprisingelectrodepositing chromium on the article by passing a current of A to 3a. s. i. to said article as cathode in an aqueous chromium plating bathat a temperature of 90 to 130 F., said bath having a C1'O3 content of150 to 300 g./l. and containing dissolved sulfate, SO4=, as catalyst,the ratio of CrOz to dissolved sulfate being 85:1 to 40:1, platingchromium on the article to form a deposit about 2 to 15 mils thick, saiddeposit being characterized by having 1200 to 10,000 crack-lines perinch and a stress of to 20,000 p. s. i., and thereby producing achromium plated article of hard steel having a fatigue strength of atleast 70% of that of the original unplated steel.

4. A method for reducing the loss of fatigue strength following chromiumplating of an article of hard steel comprising electrodepositingchromium on the article by passing a current of A to 5 a. s. i. to saidarticle as cathode in an aqueous chromium plating bath at a temperatureof 90 to 110 F., said bath having a CIOs content of 300 to 400 g./l. andcontaining dissolved sulfate, SO4=, as catalyst, said bath containingcalcium sulfate as the source of said dissolved sulfate, the amount ofcalcium sulfate being sutficient to saturate the bath and to provide anundissolvcd excess of calcium sulfate, said bath also containing asoluble calcium compound in an amount sufficient to suppress theconcentration of dissolved sulfate from the unsuppressed saturationconcentration of the latter to a lower value corresponding to a ratio ofCrOs to dissolved sulfate of 100:1 to 40:1, plating chromium on thearticle to form a deposit at least 1 mil thick, said deposit beingcharacterized by having at least 1000 crack-lines per inch and a stressbelow +5,000 p. s. i., and thereby producing a chromium plated articleof hard steel having a fatigue strength of at least of that of theoriginal unplated steel.

5. The method of claim 1 wherein, prior to chromium plating, a copperstrike is deposited on said steel article to minimize any embrittlementof the steel article in said chromium plating bath.

6. The method of claim 1 wherein the steel of said upplated article hasa hardness of at least 33C Rockwell.

7. The method of claim 1 wherein the fatigue strength of the steel ofsaid unplated article is 60,000 to 110,000 p. s. 1.

8. The method of claim 1 wherein said chromium deposit is at least 5mils thick.

9. The method of claim 1 wherein said chromium plated article of hardsteel has a fatigue strength of at least of that of the originalunplated steel.

10. The method of claim 1 wherein said sulfate, SOE, comprisessubstantially the entire catalyst in said bath.

11. A method for reducing the loss of fatigue strength followingchromium plating of an article of hard steel comprisingelectrodepositing chromium on the article by passing a current of A to 5a. s. i. to said article as cathode in an aqueous chromium plating bathat a temperature of to F., said bath having a CrOz content of 300 to 400g./l. and containing dissolved sulfate, SO4=, as catalyst, said bathcontaining calcium sulfate as the source of said dissolved sulfate, theamount of calcium sulfate being sufficient to saturate the bath and toprovide an undissolved excess of calcium sulfate, said bath alsocontaining a soluble calcium compound in an amount sufficient tosuppress the concentration of dissolved sulfate from the unsuppressedsaturation concentration of the latter to a lower value corresponding toa ratio of CrOa to dissolved sulfate of 85:1 to 40:1, plating chromiumon the article to form a deposit at least 1 mil thick, said depositbeing characterized by having at least 1000 crack-lines per inch and astress below +5,000 p. s. i., and thereby producing a chromium platedarticle of hard steel having a fatigue strength of at least 60% of thatof the original unplated steel.

References Cited in the file of this patent

1. A METHOD OF REDUCING THE LOSS OF FATIGUE STRENGTH FOLLOWING CHROMIUMPLATING OF AN ARTICLE OF HARD STEEL COMPRISING ELECTRODEPOSITINGCHROMIUM ON THE ARTICLE BY PASSING A CURRENT OF 1/4 TO 5 A.S.I.TO SAIDARTICLE AS CATHODE IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATUREOF 90 TO 110* F., SAID BATH HAVING A CRO3 CONTENT OF 100 TO 400 G./L.AND CONTAINING DISSOLVED SULFATE, SO4=, AS CATALYST, THE RATIO OF CRO3TO DISSOLVED SULFATE BEING 100:1 TO 40:1 PLATING CHROMIUM ON THE ARTICLETO FORM A DEPOSIT AT LEAST 1 MIL THICK, SAID DEPOSIT BEING CHARACTERIZEDBY HAVING AT LEAST 1000 CRACK-LINES PER INCH AND A STRESS BELOW +5,000P.S.I., AND THEREBY PRODUCING A CHROMIUM PLATED ARTICLE OF HARD STEELHAVING A FATIGUE STRENGTH OF AT LEAST 60% OF THAT OF THE ORIGINALUNPLATED STEEL.